There exists a broad range of potential applications for the in vivo delivery of electrical waveforms, including bone repair, dermal wound healing, nerve regeneration, transport and delivery of therapeutic agents, and the like. Certain known techniques utilizing such electrical waveforms are generally referred to as iontophoresis, electro-osmosis, electroporation, and electropermeabilization. As there is limited understanding of the phenomenon known as electroporation, the terms electroporation and electropermeabilization are often used interchangeably.
Iontophoresis involves the application of electric currents to drive or repel oppositely charged particles through tissue. Iontophoretic devices have been known since the early 1900's. U.S. Pat. Nos. 3,991,755; 4,141,359; 4,398,545 and 4,250,878 disclose examples and some applications of such devices.
Electro-osmosis occurs when an electric field is applied parallel to a charged surface in contact with a solution. The net ion movement causes a hydrodynamic flow of all molecules which are nearby in the solution (Dimitrov et al. (1990)). During electroporation, the transport of therapeutic agent across the cell membrane is commonly believed to involve diffusion, the movement of molecules from areas of higher concentration to areas of lower concentration. This transmembrane movement into the cytosol of electroporated cells may be enhanced by electro-osmosis (Sowers (1992) pp. 120-125).
Electroporation refers to the application of electric fields of sufficient intensity and duration as to induce transient increases in cell membrane permeability. The cell membrane is a selectively permeable barrier that greatly inhibits the penetration of many therapeutic agents into the cytosol. As a comparative example, Mir et al. (1992) report a 10,000-fold increase in the cytotoxic activity of bleomycin, a normally impermeable chemotherapeutic agent, in the electroporated cells.
Rols et al. (1990) describe electroporation as a threshold dependent phenomenon in that electric field intensity must be higher than a critical threshold to induce cell permeability. They further report that the extent and duration of membrane permeabilization is dependent on pulse duration and number. Provided that the electric field strength was not too high and the pulse duration not too long, electroporation of the cell membrane appears reversible (Zimmerman (1986) pg 177). Thus there exists an opportunity for the use of electroporation of cell membranes to achieve therapeutic benefits. In order to achieve success, the electric fields propagated in tissue by the delivery of specific electrical waveforms must apply sufficient transmembrane voltage and pulse duration to induce cell membrane permeability, yet not exceed inherent upper limits leading to cell lysis (death).
Other than transdermal or transcutaneous applications, the in vivo electroporation of cell membranes is a relatively limited field. In U.S. Pat. No. 5,273,525 to Hofmann, and U.S. Pat. No. 5,389,069 to Weaver, a description is given of a two electrode system for acute placement during tissue electroporation.
Nishi, et al. (1996), Ceberg, et al. (1994), Salford, et al. (1993) and Okino, et al. (1987) also describe two electrode systems for tissue electroporation. Where described therein, the electrodes are of rod type (needle) construction, acutely placed in tissue, and spaced approximately 0.5-2 cm apart. However, the prior art does not provide clear guidance as to the need for uniformity in the electric field propagation. Additionally, there are not known to be methods for the confinement of threshold level field intensities to the targeted tissue.
As depicted in FIG. 1, panel A, the electric field propagated in tissue by a two electrode system 20a-20b as described in the prior art would be considerably weaker in the region of tissue that is proximal to the midpoint between the electrodes (represented as the dashed box 22). In fact, the field strength in the tissue will weaken geometrically as the distance from either electrode 20a, 20b is increased (panel B), and the field strength in the mid-region of tissue will also weaken geometrically as the distance "L" between the two electrodes is increased. As electroporation is considered to be a threshold-dependent phenomenon, with inherent upper limits due to the risk of cell lysis, a two electrode system is poorly suited to establish uniform electric field coverage, i.e., uniform electroporation, in the tissue targeted for treatment.
Plate-type electrodes 24a, 24b aligned in parallel (see FIG. 2) have been proposed to provide a uniform electric field 26 for electrochemotherapy delivered transcutaneously (U.S. Pat. No. 5,468,223 to Mir), and U.S. Pat. No. 5,439,440 to Hofmann describes an electrode for in vivo electroporation as spaced apart parallel arrays of needle electrodes 28a, 28b, 28c, 28d mounted on a dielectric support member 30 (see FIG. 3). This design allows adjustments in needle depth and spacing between the parallel arrays (separated by distance L), but not the spacing of electrodes within each array 32. As disclosed, the array design has features that suggest similarity to the plate-type electrodes of FIG. 2. However, in order to approach electric field uniformity (field 26 in FIG. 2), the spacing 32 between adjacent electrodes 28a, 28b in the same array would need to be in the range of 0.25 L or less. The anatomy of the tissue involved may limit the number of electrodes that can be spaced within the targeted region. If the spacing between adjacent electrodes on the array is much greater than 0.25 L, then the same field uniformity problems noted previously will remain for this array design. Furthermore, although the needle arrays are described as adjustable in depth, there is no disclosure providing a means to confine the field effects for deep tissue applications in the third dimensional axis relative to the electrode arrays.
Thus, it would be desirable to provide a means to propagate electric fields of adequate intensity for a three dimensional region of tissue so as to achieve uniform electroporation, while mitigating electric field-induced cell lysis. In addition, there is a need to confine such therapeutic field effect to the targeted region of tissue.
It is also considered desirable to provide the means to implement effective in vivo therapies utilizing electroporation that rely on: (i) adequate transmembrane voltages, i.e., adequate electric field intensities, propagated throughout a predetermined region of tissue, (ii) minimization of cell lysis due to electrical waveform delivery, most likely to occur in the electrode milieu where electric field intensities are the greatest, (iii) confinement of therapeutic electric field effects to the predetermined region of tissue, and (iv) adequate concentration of therapeutic agents in the extracellular space of the cells in the tissue being treated.